The present invention relates generally to methods for conditioning flue gas from coal-burning boilers to facilitate the removal of fly ash from the flue gas. The present invention relates more particularly to flue gas conditioning methods employing the injection of sulfur trioxide (SO.sub.3) into the flue gas to render the fly ash more susceptible to removal by an electrostatic precipitator.
Coal is combusted with air in a boiler to generate heat which, in turn, generates steam which, in turn, powers a turbine to generate electricity. Coal contains sulfur. When coal is combusted, the products of combustion include fly ash and sulfur dioxide (SO.sub.2) which are exhausted from the boiler through a flue as part of an exhaust stream known as flue gas. Fly ash and SO.sub.2 are both undesirable pollutants and must be removed from the flue gas to a desirably low level, usually set by environmental regulators.
One approach to removing fly ash is to employ an electrostatic precipitator through which the flue gas flows prior to being exhausted into the atmosphere. The efficiency with which fly ash is removed from the flue gas by the electrostatic precipitator depends in part upon the electrical conductivity of the fly ash. This, in turn, is influenced by the absorption, by the particles of fly ash, of sulfuric acid which is generated as a by-product of the combustion process through the reaction of sulfur dioxide with air and water in the flue gas. When the flue gas contains relatively large quantities of SO.sub.2, there is a comparably large quantity of sulfuric acid produced in the flue gas, and the amount of sulfuric acid available to precipitate upon the fly ash particles is sufficient to produce relatively efficient removal of fly ash at the electrostatic precipitator.
In coal having a relatively large sulfur content, only a portion of the SO.sub.2 generated by combustion is required for conditioning the fly ash. The rest of the SO.sub.2 is excess. Large amounts of excess SO.sub.2 in flue gas exhausted to the atmosphere are undesirable because SO.sub.2 can cause pollution problems, such as acid rain, absent expensive expedients for removing SO.sub.2 from the flue gas. To reduce the amount of sulfur dioxide generated by the combustion process, boiler operators have been switching to coal having a relatively low sulfur content. However, the combustion of low sulfur coal not only reduces pollution problems due to excess SO.sub.2, but also the amount of SO.sub.2 produced by the combustion process is then not sufficient to produce the quantities of sulfuric acid required to efficiently remove fly ash at the electrostatic precipitator. To combat the problem described in the last sentence, boiler operators have been generating SO.sub.3 extraneously for injection into the flue gas to combine therein with air and water from the flue gas to form sufficient sulfuric acid to precipitate upon the particles of fly ash and provide the necessary efficiency for electrostatic removal of the fly ash from the flue gas.
In one general type of extraneous SO.sub.3 generator, sulfur and air are reacted in a sulfur burner to produce a first mixture comprising sulfur dioxide and air, the sulfur dioxide in the first mixture is converted to SO.sub.3 in a catalytic converter to produce a second mixture comprising SO.sub.3 and air and that mixture is then injected into the flue gas. Many conventional SO.sub.3 generators of this type produce a mixture of SO.sub.3 and air having an SO.sub.3 concentration of about 1-2%, in turn produced from a first mixture of SO.sub.2 and air having an SO.sub.2 concentration similarly of about 1-2%. As used herein, when SO.sub.2 and SO.sub.3 contents are expressed as per cents, they are volume per cents.
Efficient conversion of SO.sub.2 to SO.sub.3 in a catalytic converter generally requires that the SO.sub.2 entering the catalytic converter be within a predetermined temperature range, e.g. in the range 780.degree.-850.degree. F. (416.degree.-556.degree. C.). When the concentration of SO.sub.2 in the mixture of air and SO.sub.2 produced by the sulfur burner is relatively low (e.g. 1-2%), the temperature of that mixture may not be sufficiently high to satisfy the minimum temperature requirement necessary for a mixture of SO.sub.2 and air entering a catalytic converter or even to enable complete combustion of the sulfur in the sulfur burner. The low temperature in a mixture of air and SO.sub.2 having a low SO.sub.2 concentration is due to the cooling effect of the relatively large volume of air in such a mixture. For example, assuming one desires a given quantity of SO.sub.2, if the concentration of SO.sub.2 in the mixture of SO.sub.2 and air is 1%, the volume of air in that mixture is four times greater than the volume of air in a mixture of air and SO.sub.2 having a SO.sub.2 concentration of 4%.
In order to assure that the mixture of air and SO.sub.2 produced at the sulfur burner has the minimum desired temperature, it has been conventional to preheat the air introduced into the sulfur burner. The preheating operation is usually conducted in an electric heater, and this consumes relatively large quantities of electric energy which, in turn, is relatively expensive.
Another drawback to the employment of a mixture of SO.sub.2 and air having a relatively low SO.sub.2 concentration is that, because of the relatively large air volume, the processing vessels and conduits required to accommodate the large gas volume are themselves relatively large. This increases the expense of the processing equipment; and, in addition, there are increased heat losses with larger-sized processing equipment. Increased heat losses, of course, require additional preheating of the air, in turn, expending additional energy and entailing increased operating expense.